Filter cigarettes with means for diluting the smoke stream with outside air have been constructed in the art. For example, the wrappers surrounding the filter, the tobacco, or both may be made of a porous material that allows outside air to be drawn into the smoke stream, thereby diluting it. Alternatively, either or both of these wrappers may be perforated. It is also known to provide grooves or flutes in the filter or in a filter mouthpiece that direct dilution air to the smoker's mouth.
In one configuration, shown in U.S. Pat. No. 3,490,461, a fluted plastic mouthpiece is provided, wherein the dilution air enters perforations in the tipping paper and flows down the flutes to the smoker's mouth. In this configuration, the dilution air first mixes with the smoke in the smoker's mouth. Another structure is shown in U.S. Pat. No. 4,256,122, in which the dilution air flows through grooves embedded in a filter previously wrapped in a nonporous wrapper.
A problem with structures including perforations in flow communication with flutes or grooves that deliver dilution air directly to the smoker's mouth without first mixing with the tobacco smoke stream is that the perforations may become occluded or blocked by the smoker during actual smoking. In this case, the amount of dilution air in each puff is reduced or eliminated, thus increasing the amounts of tar and nicotine delivered to the smoker in each puff. This problem may be alleviated by the use of a secondary source of dilution air. The secondary source is provided so that the dilution air flows through channels that cannot be occluded by the smoker in addition to flowing through the primary dilution grooves. U.S. Pat. No. 4,256,122 shows a filter configuration that has two sets of channels.
The foregoing known dilution means is effective to direct through one set of channels dilution air to the smoker's mouth while directing through the other set of channels dilution air into the tobacco column filter interface and back to the smoker's mouth but provides over-dilution of the smoke delivered to the smoker's mouth in many instances. For example, air admitted through perforations chosen to provide a specified dilution in the event primary dilution channels are completely occluded will cause over-dilution when such perforations are not occluded or are only partially occluded. Also, when this dilution means is employed with discrete rows of perforations experience has shown that the size of such perforations required for desirable values of secondary dilution leads to undesirable smoke blow-back through these perforations.
Accordingly, a filter cigarette that incorporates both primary and secondary dilution channels while simultaneously minimizing over-dilution and smoke blow-back is desired.